Allow modules to use other modules (#6162)

* Allow private imports inside modules

Can call `use ...` inside modules now.

* Add more tests

* Add a leak test

* Refactor exportables; Prepare for 'export use'

* Fix description

* Implement 'export use' command

This allows re-exporting module's commands and aliases from another
module.

* Add more tests; Fix import pattern list strings

The import pattern strings didn't trim the surrounding quotes.

* Add ignored test

crates/nu-command/src/default_context.rs

@@ -42,6 +42,7 @@ pub fn create_default_context() -> EngineState {
             ExportDefEnv,
             ExportEnv,
             ExportExtern,
+            ExportUse,
             Extern,
             For,
             Help,